A holistic performance analysis and classification of multiport antennas (MPAs) is conducted in this paper. We focus on 5.9 GHz vehicle-to-vehicle communications suited to the emerging technology of intelligent transportation systems. Three-dimensional (3-D) uniform/isotropic, directional, and omnidirectional propagation scenarios are considered to account for any wireless environment. The presented analysis can be adapted to any MPA with an arbitrary number of ports, operating in any frequency band, or used in other emerging technologies such as in 5G and beyond 5G communications. On top of the classical key performance metrics (KPMs) in communication theory, i.e., the diversity antenna gain (DAG) and channel capacity (CC), we employ for the first time the energy efficiency as one more KPM capable to characterize performance and classify MPAs, particularly when DAG and CC fail to do so. Computation of the aforementioned KPMs departs from a covariance matrix formulation incorporating all intrinsic features that affect MPA performance, namely, MPA radiation characteristics, MPA termination conditions, and wireless propagation channel attributes. Accordingly, we derive the ideal form of the covariance matrix under the standardized and widely adopted 3-D uniform/isotropic wireless propagation scenario. A good MPA design should be one with a covariance matrix as close as possible to this ideal one. The adopted performance analysis methodology can thus inform the design of optimum MPAs and accordingly, we designed a proof-of-concept box-shaped MPA which shows outstanding performance across all propagation scenarios. It would be wise to conduct similar performance analyses as in this paper before releasing an MPA design.